Propellers or fan blades are subject to high bending stresses at the roots thereof while rotating at low rotational speeds. Bending stresses at the roots are a problem during low rotational speeds because the centrifugal forces generated by the rotary motion are insufficient to straighten the blade radially. To reduce these stresses it is known in the art to use pinned blade roots. Examples of pinned roots appear in U.S patent application Ser. No. 201,232, filed Jun. 2, 1988, and assigned to the same assignee as the present invention. Such a blade may be attached at its root to a rotating engine structure by a clevis-pin arrangement that provides a predetermined amount of circumferential blade moveability. A pinned root provides flexibility at the root of the fan blades and therefore relieves the bending stresses generated during rotation. While the use of pinned blade roots solves the above-mentioned problem, they create a separate one of their own. Thus, a pinned blade can rotate about the axis of the blade root pin between the extremes allowed by the pinned root in a manner analogous to a pendulum. The flexibility of a pinned root, which is desirable for reducing bending stresses, permits excessive vibrations, oscillations or relative motions of the entire fan blade that may deteriorate the engine cowling and the engine structure adjacent to the blade roots. These motions, which for convenience will be collectively referred to as vibrations, are variously known in the art by the noise created thereby, that is, as a "chatter," "clank," or "hammer." "Chatter" is a problem only at low rotational speeds, such as during windmilling--the low speed turning of the blades caused by the wind. That is, unlike high rotational speeds where the rotationally generated centrifugal force is sufficient to hold the blades radially outward from the axis of rotation and to prevent thereby the secondary rotation of the fan blade between the extremes allowed by the pinned blade root, at low speeds each blade is able to move within its individual rotational arc centered on its pin axis.
Chatter can be an even greater problem when the blades have a high aspect ratio. When operating at high rotational speeds, the centrifugal field operating on the blades is sufficiently strong enough to keep the blade dips from moving circumferentially with respect to each other such that they touch. At low rotational speeds, other forces, such as wind gusts and gravity, are strong enough to overcome the centrifugal force and cause the blades to move circumferentially with respect to each other. Thus, in such blade designs it is possible for the tips of the blades to touch during chattering, thereby creating the potential for damaging the blades in general and the blade tips in particular, both of which are obviously undesirable results.
Furthermore, during low speed rotation of the fan blades each fan blade rotates at a unique instantaneous angular speed due, for example, to differing air foil inertia or to wind gusts. Thus, the load on each pinned blade root varies from blade to blade. In turn, this stresses the blade mounts and can lead to their damage as well as to damage to the actual blade roots.
Thus, it would be desirable to provide a pinned root fan blade that was not subject to the debilitating effects of vibration and unequal loading that occurs during low rotational speeds.